fix(package): update yargs to version 15.0.1

Closes #63

Author: greenkeeper[bot]

package.json

@@ -34,7 +34,7 @@
     "picture-tube": "^1.0.0",
     "request": "^2.75.0",
     "request-promise": "^4.1.1",
-    "yargs": "^8.0.1"
+    "yargs": "^15.0.1"
   },
   "devDependencies": {
     "finalhandler": "^1.1.0",